Means for continuously treating strip



Oct. 13, 1964 A. V. ALEXEFF ETAL MEANS FOR CONTINUOUSLY TREATING sma 3 Sheets-Shae: 1

Filed Sept. 14, 1962 REVERSING GEAR BOX INVENTORS ALEXANDER V. ALEXEFF HOWARD R. RICHARDS ZOBENICA BY MIL N zwa/ A. v. ALEXEFF ETAL 3,152,794

MEANS FOR CONTINUOUSLY TREATING STRIP 3 Sheets-Sheet 2 Oct. 13, 1964 Filed Sept. 14, 1962 MOTOR GENERATOR FIELD EXGITER MOTORA SPEED REDUCER SPEED REDUCER MOTOR GENERATOR FIELD EXCITER 386 390- 400. ED M 4m?- 4 i r I4o i MOTOR I l5q SPEED REDUCER' l A Y Y I 250 240 46 I REVERSING l MOTOR 33 SPEED l REDUCER l l FIG 2A I A V l INVENTORS ALEXANDER v ALEXEFF I60 HOWARD R. RICHARDS BY MIL N ZOBENICA Oct. 13, 1964 Filed Sept. 14, 1962 A. V. ALEXEFF ETAL MEANS FOR CONTINUOUSLY TREATING STRIP 3 Sheets-Sheer 3 MOTOR SPEED REDUCER\ J I5 i a I 3| 250 240 I s8 I REVERSING MOTOR l SPEED I I REDUCE CONTROL I POTENTIOMETER l 260 I I |9 30 I I W I 8d W 2..- 1% I {2% 52d H6O I00 3| a 3| 1 5i I90 FIG. 2

FIG. 3

INVENTORS ALEXANDER V. ALEXEFF HOWARD R. RICHARDS BY MILAN ZOBENICA United States Patent 3,152,794 NEANS FUR CONTHNUQUSLY TREATING STRIP Alexander V. Alexefi, Cleveland, Howard R. Richards, Lakewood, and Milan Zohenica, Parana, Ohio, assignors to Industrial Ovens, Incorporated, Cleveland, Ohio, a corporation of Ohio Filed Sept. 14, 1962, Ser. No. 223,727 6 Claims. (Cl. 263- 3) The present invention relates to the treatment of webs and strands and particularly to means for attaining precise and responsive temperature control of the treated material. For purposes of this application, strands is to be understood to include wires, cords, monofilaments, narrow tapes and tubings, and strips is to be understood to include strands and also webs such as metal strip or sheet, paper, textiles, films and wide tapes. The invention presently appears to have most promise in the treatment of metal strip, but may be advantageously employed wherever strip material is to be heated in the absence of contact with rolls or other supporting structure.

In numerous present-day continuous or semi-continuous processes for the heat treatment of strip, it is desirable to maintain a high degree of uniformity of treatment of the strip from increment to increment of its length. This is particularly desirable where the treatment can be wholly satisfactory only when carried out within narrow critical ranges of conditions. Any variables which tend to displace strip temperature across critical limits must, therefore, be counteracted by adjustment of the rate of heat transfer to the web. Such adjustment may be attempted in a number of ways. For example, temperature may be varied by increasing or decreasing the output of the burners or heating elements which are the source of heat. It will be apparent that close strip temperature control under quickly fluctuating control conditions cannot be practically attained with this rudimentary arrangement. Another procedure is to provide a standby supply of heat (i.e., heated air) which can be supplied to the oven or drier on demand. Conversely, an overheated oven may be provided which is cooled to the desired temperature by outside air or other cooling medium, the rate of supply of the cooling medium being varied in response to control requirements to counteract any tendency of the strip to exceed temperature limitations. Thus, a degree of strip temperature control may be attained by responsively and continuously damping into the oven or drier an accumulated heating or cooling medium. However, the inherent waste and expense of this procedure renders it impractical for close temperature control. If a relatively wide temperature range is allowed, damping in of the alternative supply used occurs only relatively infrequently, but for close control dampingin of the alternative heating or cooling medium must occur almost continuously, representing a prohibitive heat loss. Another possibility is to vary web speed, but in most production set-ups, to continuously vary speed of the entire processing line is obviously impractical.

The above considerations limit the possibilities of many strip heat treating processes where the ideal would be to closely approachicritical temperature without exceeding it. An example is the continuous heat treatment of ferrolls and non-ferrous metallic strip or sheet Cold worked.

steel strip andsheets are almost always subjected to a heat treatment to effect changes in-mechanical properties. In the as-rolled state, cold reduced strip has little ductility and must be heat treated to restore the loss of ductility and to permit further working of the steel. It is sometimes necessary' to subject hot rolledistrip to a final normalizing heat treatmentif the steel leaves the last finishing stand at a temperature below the upper critical value.

For most applications today, steel strip is heat treated by box annealing techniques, where coiled or stacked sheets are placed in a bell-type furnace and subjected to a long heat treating cycle by varying the temperature of the atmosphere within the furnace. Box annealing produces a dead-soft, fully recrystallized material and close control of the temperature is not a critical factor because of the long treating cycle.

Because box annealing is relatively inflexible as to the nature of the treating cycle and since long heat treating cycles are incompatible with the continuous and rapid production of single lengths of steel strip and sheet thousands of feet long, continuous annealing techniques are desirable and, in many cases, necessary for the production of quality materials. Continuous or semi-continuous strips or sheets of ferrous or non-ferrous material are rapidly passed through a furnace at line speeds of several hundred feet per minute. To save floor space, the strip is usually festooned within the furnace.

Cold reduced carbon steel may be continuously annealed in such a furnace by supplying sufiicient heat to the strip to heat the strip to a temperature slightly above its lower critical. The heat treatment may be conducted in a controlled or protective atmosphere, particularly if the steel is to be tin plated. In many applications it is not practical or possible to continuously heat treat metals in a furnace having festooning rolls. This is particularly true where the treating temperature approaches or exceeds 2000 F. and the extremely high temperature shortens the life of the rolls. Furthermore, specialized metals, such as iron-silicon electrical sheets, may be adversely affected by contact with a roll during the annealing cycle.

To overcome these problems, free loop or catenary type furnaces have been developed. In furnaces of this type the sheet or strip is continuously fed through a furnace, but is not contacted by rolls or supporting structure within the heating zone. In continuous furnaces of the catenary or free loop type, each increment of strips is in the furnace for a relatively short time. During this time, each increment of strip must be heated to a temperature which is sulficiently high to recrystallize that'increment, but which is not high enough to induce deleterious grain growth. Since grain growth is proportional to the excess of heat beyond that required for recrystallization and to the time of exposure before cooling, it is desirable to control both of these variables while uniformly recrystallizing each increment of the strip.

Heretofore it was necessary to compromise quality and uniformity by choosing operating conditions sothat the strip temperature was well above the recrystallization point despite the desirability of limiting grain growth.

An object of the present invention is to provide means for maintaining constant strip temperature within a free loop or catenary furnace despite fluctuations in strip speed, temperature of the input side of the strip, and any other variables that tend to affect strip temperature and exposure time. i

A further object of this invention is to avoid having to vary oven temperature in order to accommodate desired changes in strip speed. Such variation of oven temperature may adversely affect the refractory lining and/or shell of the oven. The invention makes possible far more rapid accommodation of desired changes in strip speed than is possible (within economic reason) by varying oven temperature.

The objects and advantages of the invention will become apparent from the following description of specific components in each illustrated embodiment are, therefore, illustrated schematically in the interest of clarity in order that the invention itself may be most concisely and completely disclosed and understood.

The drawings are partly schematic and partly diagrammatic illustrations of a continuous or semi-continuous strip processing system employing the invention.

In the drawings:

FlGURE 1 is an elevational view of a catenary or free loop type strip processing system employing the invention.

FIGURE 1A is a View similar to FIGURE 1 showing certain changes in elements of the system of FIGURE 1.

FIGURE 2 is an elevational view of a catenary or free loop type strip processing system according to another aspect of this invention.

FIGURE 2A is a view similar to FIGURE 2 showing certain changes in elements of the system of FIGURE 2.

FIGURE 3 is a schematic 'view of an elongated arrangement of the cooling chamber shown in FIGURE 2.

In FIGURE 1, the strip which is to be subjected to a heat treatment is indicated by the reference numeral 19. The heating means, according to one aspect of the present invention, comprises a chamber Iii into which the strip llii is looped. For this purpose the chamber 11 is diagram- 4 ming device 20 will have the same directional characteristics as the power output from the motor 14 and the speed reducer 15.

A second control input 23 is an overdrive-underdrive input having a rotational movement which is proportional to the rotation imparted by a variable speed reversing motor 24 through a speed reducer 25.

The rotation of the reversing motor 24 is controlled in response to temperature sensed by a radiation pyrometer 2'7 through a suitable and conventional control potentiometer 26. The radiation pyrometer is positioned within the chamber 11 to sense web temperature at the output end of the chamber 11.

matically illustrated as being open at the top, but suitable baffles (not shown) may be provided at the top of the chamber 11 to conserve heat, protect the rolls, and confine any controlled or protective atmosphere that may be used.

Heat may be supplied to the chamber ill by suitable heating means (not shown) such as electric heating elements, gas burners, radiant tubes, and the like. The particular heating means selected depends primarily upon the type of strip being treated. Since the device according to this invention may be employed to heat treat metals to temperatures up to and in excess of 20il0 F., the chamber ill may be internally lined with a suitable refractory material (not shown).

The strip Ill is fed into the chamber 11 bypinch rolls I2 and finally passes between pinch rolls 13. The pinch rolls 1.2 and 13 are driven in synchronism by a variable speed DC. motor 14 and an associated speed reducer 15. Between the pinch rolls 12 and 13 the strip 19 is supported and driven by pinch rolls 16. The strip 16 is supported and driven by the rolls 12, 13, and i6 and vertical catenary loops are formed on either side of the pinch rolls 16.

A first catenary loop 17 is formed by the strip 19 between the rolls 12 and 16 and extends into the treating chamber H. A second catenary loop 18 is formed by the strip 10 between the rolls in and 13. Depending'upon the treatment towhich the strip is subjected, an'accumulating chamber 19 may be provided on the outfeed side of the chamber 11 to enclose the second catenary loop 18. The chamber 19 may serve to quench or cool the strip 10, as by being provided with suitable spraynozzles or a suitable cold gas supply (not shown). If desired, the cham ber 19 may contain a coating or pickling bath.

The relative lengths of the first and second catenary loops 1'7 and 18 are determined by the difierence in example, constitute the means described in the following paragraphs. v

The rotational movement of the pinch rolls 16 is determined by the output of a mechanical summing device 20. The summing device 20 has two inputs. A first power input 21 transmits the rotational movement of the pinch rolls 12 and 13, Since the output of the summingdevice which is shown reverses the rotational direction of its power input 21, a one-to-one ratio reversing gear box 22 is provided to insure that the power output of the sum- V by the reference numeral 10a.

The power output of the summing device 20 is the algebraic sum of the inputs 21and 23. The potentiometer is initially adjusted to have a Zero output at a predetermined temperature sensed by the radiation pyrometer 27. At zero output of the potentiometer, the motor 24 is not called upon to move. When the sensed temperature falls below the predetermined value, the potentiometer generates an output of a first polarity causing the motor '24 to turn in a first direction. When the sensed temperature rises above the predetermined value, the potentiometer generates an output of a second polarity causing the motor 24 to turn in a second direction.

The control potentiometer 26 is adjusted to have zero output at the desired strip temperature. If the strip which has traveled through the furnace is sensed by the pyrometer 2'7 as having an exit temperature corresponding to the selected value, the output of the summing device 20 will correspond to its input 21 and the pinch rolls 16 will be driven in synchronism with the pinch rolls 12 and 13. If the heat imparted to the strip and sensed by the pyrometer 27 should vary from the selected temperature, the second input 23 will either add or subtract a certain measure of rotation to or from the normally synchronized rotative movement of the rolls 16 to thereby, respectively, decrease or increase the length of the catenary loop 17. Turning the input 23, a given rotative direction and amount will effect the same adjustment in the length of the catenary loop whether the motor 14 and the strip line is running fast or slow or not at all. When the line is running, if the sensed temperature is more than the selected temperature, the input 23 will be added to the input 21 by the summing device 20 to shorten the ca'tenary loop 17 and lengthen the catenary loop 18 so that less strip will be exposed to the heating chamber 11 and the sensed strip temperature will decrease to the selected temperature. Conversely, if the sensed temperature is less than the selected temperature, the input 23 will be subtracted from the input 21 by the summing device 20 to lengthen the catenary loop 17 and shorten the catenary loop 18 so that more strip will be exposed to the heating chamber ll. and the sensed strip temperature will increase to the selected temperature.

If production demands require a greater line speed of the web It the selected temperature will be uniformly and automatically maintained. Thus, if the line speed is increased or decreased, the pyrometer 27 will sense the resulting increase or decrease in web temperature and will automatically adjust the catenary loops to proper exposure lengths] 7 V FIGURE 2 illustrates another aspect of this invention having another'form of control setting means included by the overall control system for driving and guiding the web. .In this arrangement the catenary loop length is varied by a dancer roll. Referring to FIGURE 2, the Web which is to be subjected to heat treatment is indicated The heating means, according to this aspect of the invention, comprises a chamber lla'into which the web. 10a is looped. The chamber 11a may be similar to the chamber 11.

The web 10a is fed into the chamber 11a by pinch rolls 12a and finally passes between pinch rolls 13a. The

pinch rolls 12a and 13a are driven in synchronism by a variable speed DC. motor 14a and an associated speed reducer 15a. Between the pinch rolls 12a and 13a the Web a is supported for vertical translation by dancer rolls 16a. The web ltla is driven by the rolls 12a, 13a and 16a, and vertical catenary loops are formed on either side of the dancer rolls 16a. A first catenary loop 17a is formed by the web 10:: between the rolls 12a and 16a and extends into the treating chamber 11a. A second catenary loop 18a is formed by the web 19a between the rolls 16a and 13a.

The dancer rolls 1611 are mounted for lateral displacement so that they may be shifted up or down in a vertical path. For this purpose the ends of the dancer rolls 16a may be mounted in blocks which are engaged by an endless cable or chain 30 at either side of the blocks. Each cable or chain 30 passes around associated sprockets or sheaves 31, one of which is powered by a reversible motor24a through a speed reducer 25a. There is a sliding connection between the sprocket 28a and shaft 29a and these elements are keyed for rotation together. The rolls 16a are thereby driven through the linkage 32a but vertical shifting of the rolls 16a is accommodated.

, A Web temperature detector in the form of a radiation pryometer 27a is provided to sense web temperature at the output end of the chamber 11a. Through a suitable and conventional control potentiometer 26a the motor 24a is controlled in response to sensed temperature in a manner similar to the'control of the motor 24. Upon an increase in temperature of the web, the dancer rolls 16a are raised to shorten the lengths of the catenary loops 17a and 18a, and upon a decrease in temperature, the dancer rolls 16a are lowered. If desired, a cooling chamber 19 may be provided to enclose the path through which the rolls 16a are translated. This cooling chamber as well as the chamber 11a may be provided with a protective or controlled atmosphere. The cooling chamber 19a may be elongated vertically by any desired amount to provide sufficient cooling travel for all lengths of loop. In FIGURE 2, the cooling provided by 19a might well be wholly inadequate at the illustrated condition. The illustrated condition is near the critical condition with regard to cooling adequacy, since the loop 17a is near its greatest length and the. rolls 16a are near their lowest points. However such inadequacy would be remedied by revising the relative vertical dimensions of the parts by elongating the cooling chamber 19a so that, near the critical, condition, the parts would have the relationship schematically illustrated in FIGURE 3.

It is to be noted in FIGURE 3 that the strip does not contact any roll until it has passed through a considerable cooling distance. This can be increased to any desired amount by further vertically elongating the chamber 19a.f v Theloop 18a counterbalances the loop 17a. However the loop 1 18a may be eliminated or greatly reduced in size in someapplications.

We are aware of prior proposals for controls to maintain a catenary loop within a furnace at a predetermined length. None of these prior arrangements provided means for controllably adjusting the length of the eatenary loop to any one of an infinite number of values within a range of values. However, as above described, such means is provided for applicants loops 17 and 17a.

' 1 In the systems illustrated in FIGURES 1 and 2 this means includes reversible motors 24, 24a which are in turn governed by the control potentiometers 26 and 26a according to sensed strip temperature. However, the motors 24, 24a. may be governed simply by manually operated forward: ti-reverse switches, with the rotative position of the' input 23, or the vertical position of the dancer rolls 16a," arranged to govern line speed. That is, line speed may be tied into the rotative position of the input 23, or the vertical position of the dancer rolls 1611, as through linkages and controls equivalent to the 'link-age'146 and armature voltage rheostat 149 in US.

6 Patent 2,837,834, it being understood that the apparatus of FIGURES l, 2, or 3 of the present disclosure may be suitably modified to accomplish this.

For example, the apparatus shown in FIGURE 1 may be modified as shown in FIGURE 1A and, as also indicated in such figure, may comprise part of an over-all processing line system including a motor 38, generator 39, field exciter it), an overriding or governing speed control rheostat 41, and a plurality of additional voltage responsive variable speed DC. motors (not shown).

Movement of the control input 23 through a suitable motion reducer 45 controls the setting of the overriding or speed controlling rheostat 41 thereby governing the speed of the entire line.

The radiation pyrometer 27 and the control potentiometer 26 are eliminated in FIGURE 1A and in their place is provided the forward-oiI-reverse switch 33.

Thus the speed of the line as a Whole is tied into the rotative position of the input 23. Fine web temperature control is not accomplished in the sense that it is attained in the installations of FIGURES 1 and 2, that is, varying ambient conditions are not accounted for. However, rapid changes in web speed may be accomplished with: out decreasing web temperature and while maintaining a constant oven temperature.

When it is desired to vary line speed, the switch 33 is operated in the appropriate direction to increase or decrease the length of the loop. The loop length may be changed quite rapidly while maintaining a substantially constant rate of heat input to the web by simultaneously varying the over-all line speed through the overall control rheostat 41 to maintain constant the ratio of web speed over loop length-that is, to maintain constant the exposure time within the heating chamber.

FIGURE 2A illustrates a similar change of the linkage shown in FIGURE 2 whereby the vertical position of the dancer roll 16a through the linkage 46 adjusts the setting of an over-all control rheostat 41a similar in function to the rheostat 41 of FIGURE 1A. The apparatus may comprise part of an over-all processing line system including amotor 38a, generator 39a, field ex- 'citer 40a, the over-all control rheostat 41a, and a plurality of additional voltage responsive, variable speed DC. motors (not shown).

The control potentiometer 26a and pyrometer 27a of FIGURE 2 are eliminated and replaced by a control switch 33a. Closing the switch 33a in the forward or reverse direction accomplishes a quick adjusting of the length of the catenary loop, which in turn causes the line speed to change in such a way as to maintain a constant ratio of web'velocity to loop length.

It will be seen that in the system of FIGURES 1A and 2A as well as in the systems or FIGURES l and 2, the controlmeans which acts through the support means 16 or 16a controllably adjusts the length of the catenary loop to any one of an infinite number of values within a range of values.

The heating means within the heating chamber'll or 1111 may be of a type appropriate to complete withdrawal of the web from the top of the chamber, as in the case of arrays of radiant heaters or other heating means which may be given directional heating characteristics so as to transmitheat to the web only when the web is positioned in lateral apposition thereto. In such cases, the loop 17 or 17a may be entirely withdrawn from exposure to the heating means within the chamber 11 or 11a as when the line is completely shut down. In this case it will be seen that the means for controllably adjusting the length of the catenary loop within the heating chamber comprises means tor adjusting such length to any one of an infinite number of values within a range of values starting at zero. If in this case a protective or controlled atmosphere is confined within the chamber 11 or 11:: by the previously mentioned bath-es, means (not shown) may be provided to remove the baffles to permit the loop to be Withdrawn.

While the above disclosure generally refers to heat treating and to devices which are responsive to sensed heat input in a traveling web, it will be understood that the invention is applicable to treatment procedures other than heat treatment where an agent other than heat (tor example, moisture, a solvent, or a vapor capable of depositing a film on the web) is imposed on a strip and is capable of being measured or sensed. In this connection, the term treating agent in the appended claims is to be understood to cover other agents which are equivalent to heat for the purposes of this application.

The invention is not limited to the slavish imitation of I each and every one of the details described above which have been set forth merely by way of example with the intent of most clearly setting forth the teaching of the invention. Obviously devices may be provided which change, eliminate, or add certain specific details without departing from the invention.

What is claimed is:

1. In continuous and semi-continuous strip treating apparatus, a treating station comprising a treating chamber for treating strip with a treating agent, means for driving and guiding a strip into and out of said treating chamber to form a catenary loop therein, sensing means for sensing the degree of treatment of said strip with said treating agent, means responsive to sensed deviations of said degree of treatment from a desired degree of treatment for decreasing the length of said loop in said chamber responsive to excessive degrees of treatment and increasing the length of said loop in said chamber responsive to deficient degrees of treatment.

2. In continuous and semi-continuous strip treating apparatus, a treating station comprising heating means for heating the strip, means for driving and guiding a strip into and out of said heating means to form a catenary loop therein, sensing means for sensing the temperature achieved by said strip within said heating means, means responsive to sensed deviations of said temperature from a desired temperature for decreasing th length of said loop in said chamber responsive to excessive sensed temperature and increasing the length of said loop in said chamber responsive to deficient sensed temperature.

3. In continuous and semi-continuous strip treating apparatus, spaced first and second roll means for continu ously driving said strip through a treating area, means to drive said first and second rolls in synchronism, support means for said strip between said first and second roll means, said strip forming a catenary loop between said support means and at lea-st one of said roll means, a treating chamber surrounding said catenary loop, control setting means acting through said support means for controllably adjusting the length of the catenary loop of any one of an infinite number of in-chamber length Values within a range of values, with one-to-one correspondence between- 4. In continuous and semi-continuous strip treating apparatus, a treating station comprising a treating chamber, means for driving and guiding a strip into and out of said treating chamber to (form a catena-ry loop therein, control means for said driving and guiding means, said control means including control setting means for controllably adjusting the in-clrarnber length of the catenary loop to any one of an infinite number of in-cham ber length values within a range of values, with one-to-one correspondence between each of said infinite number of in-chamber length values and each of an infinite number of control settings, so that during a treating operation there is, for each con- .trol setting, a unique iii-chamber length value and, for

each in-chamber length value, a unique control setting, said one-to-one correspondence obtaining throughout a continuous range of control settings and a corresponding continuous range ofin-chamber lengths.

5, Incontinuous and semi-continuous strip treating apparatus, a treating station comprising a treating chamber, means for driving and guiding a strip into and out of said treating chamber to form a catenaryloop therein, control means for said driving and guiding means, said control means including control setting means for controllably adjusting the in-cham=ber length of the catenary loop to any one of an infinite number of iii-chamber length values within a range of values starting at zero, with one-to-one correspondence between each of said infinite number of in-charnber length values and each of an infinite number of control settings, so that during a treating operation there is, for each control setting, a unique in-chamber length value and, for each in-cham-ber length value, a unique control setting, said one-to-one correspondence obtaining throughout a continuous range of control settings and a corresponding continuous range of in-charnber lengths.

, 6. In continuous and'semi-continuous strip treating apparatus, a heat treating station comprising a heating means, means for driving and guiding a strip into and out of said heating means to form a catenary loop therein, control means for said driving and guiding means, said control means including control setting means for controllably adjusting the length or" the catenary loop which is within the heating means to any one of an infinite number of in-chamber length values within a range of values, with one-to-one correspondence between each of said infinite number of in-chamber length values and eachof an infinite number of control settings, so that during a treating operation there is, for each control setting, a unique iii-chamber length value and, for each in-chamber length value, a unique control setting, said one-to-one corresp onden'ce obtaining throughout a continuous range of control settings and a corresponding continuous range of inchamber lengths.

References Cited in the file of this patent UNITED STATES PATENTS Germany Jan. 16, 1939 

4. IN CONTINUOUS AND SEMI-CONTINUOUS STRIP TREATING APPARATUS, A TREATING STATION COMPRISING A TREATING CHAMBER, MEANS FOR DRIVING AND GUIDING A STRIP INTO AND OUT OF SAID TREATING CHAMBER TO FORM A CATENARY LOOP THEREIN, CONTROL MEANS FOR SAID DRIVING AND GUIDING MEANS, SAID CONTROL MEANS INCLUDING CONTROL SETTING MEANS FOR CONTROLLABLY ADJUSTING THE IN-CHAMBER LENGTH OF THE CATENARY LOOP TO ANY ONE OF AN INFINITE NUMBER OF IN-CHAMBER LENGTH VALUES WITHIN A RANGE OF VALUES, WITH ONE-TO-ONE CORRESPONDENCE BETWEEN EACH OF SAID INFINITE NUMBER OF IN-CHAMBER LENGTH VALUES AND EACH OF AN INFINITE NUMBER OF CONTROL SETTINGS, SO THAT DURING A TREATING OPERATION THERE IS, FOR EACH CONTROL SETTING, A UNIQUE IN-CHAMBER LENGTH VALUE AND, FOR EACH IN-CHAMBER LENGTH VALUE, A UNIQUE CONTROL SETTING, SAID ONE-TO-ONE CORRESPONDENCE OBTAINING THROUGHOUT A CONTINUOUS RANGE OF CONTROL SETTINGS AND A CORRESPONDING CONTINUOUS RANGE OF IN-CHAMBER LENGTHS. 